1. Field of the Invention
The present invention is directed to elastomer—or rubber-based, pressure-sensitive adhesive compositions particularly useful in label and tape manufacture.
2. Description of the Related Art
During label manufacture, a laminate of a face stock, a pressure-sensitive adhesive layer, and a release liner, such as silicone-coated paper, is passed through an apparatus that converts the laminate into commercially useful labels and label stock. The converting operation processes involve printing, die-cutting, and matrix stripping to leave labels on a release liner, marginal hole punching, perforating, fan folding, guillotining and the like. It is important that the cutting action breaks the face stock and adhesive layer, but does not indent the release liner. Producing a series of labels on a backing sheet involves cutting around the label and removing the material between two labels (the matrix) while leaving the label itself attached to the backing sheet. It is important that the die-cutting machine make a clean break at operating speeds. Adhesives for these applications are formulated to have suitable viscoelastic and adhesive properties so that they can be applied to the release liner or face-stock back and will remain on the label after stripping with the required adhesion. But these properties make the adhesive film difficult to cut or break. They make die-cutting difficult and inconsistent, and cause adhesive strings and deposits on the cutting blade.
Die-cutting involves cutting the laminate through to the release liner face. Other procedures involve cutting completely through the label laminate and include hole punching, perforating, and guillotining, particularly on flat sheets.
The cost of converting a laminate into a finished product, such as a label, is a function of the various processing operations' rates. Line speed depends on whether a printing step is involved. With no printing step, e.g. computer labels, speeds can reach 300 meters/minute. Otherwise, speeds of 50-100 meters/minute are typical. While all laminate layers impact convertibility cost, the adhesive layer can limit convertibility ease. The adhesive layer's viscoelastic nature causes this limitation—its high elasticity prevents it from flowing away from the cut line during die-cutting and also promotes its transfer to cutting blades during cutting. High adhesive elasticity also causes adhesive stringiness, which hinders matrix stripping as the unwanted facing material is removed after die-cutting. High elasticity also promotes adhesive layer reconnection after the layer is severed.
Achieving good convertibility does not necessarily coincide with achieving excellent adhesive performance. Adhesives must be formulated to fit needs. Important adhesive properties include peel adhesion, tack, shear, and viscosity at various temperatures and on various substrates such as polymers, papers, glasses, and steels. Good, general-purpose adhesives may exhibit poor convertibility simply because the adhesive is difficult to cleanly sever. The adhesive may stick to a die or blade. Furthermore, within a speed range, use of a particular adhesive may result in breaking the matrix despite the fact that successful matrix stripping can occur at speeds on either side of the breaking speed. One goal is to provide adhesive systems where the adhesive has good die-cutting performance and where the matrix can be successfully stripped over the entire operating speed range.
Typical label adhesives are produced from acrylic polymer emulsions, which may be tackified by hydrocarbon or natural-resin tackifiers. While these have good die-cutting performance, they require handling large volumes of liquid and subsequent liquid removal. Accordingly, hot melt adhesives would be preferred. At low temperature, acrylic-based adhesives perform poorer than hot-melt systems. Moreover, hot melts can be used at faster line application speeds, over broader temperature ranges can have more aggressive tack, and can be used under humid conditions.
Hot-melt pressure-sensitive adhesive systems are known and consist of tackified thermoplastic elastomers such as styrenic block copolymers. For example, styrenic block copolymers containing polystyrene and polybutadiene blocks and/or polyisoprene blocks are known. These materials are generally available as pure triblocks, (sometimes referred to as SIS and SBS copolymers), and diblocks (sometimes referred to as SI and SB copolymers). The materials are also available as mixtures of diblock and triblock materials (sometimes referred to as SIS+SI and SIS+SB). Examples of these materials include elastomers marketed by Dexco and by Kraton Polymers.
It is known to use diblock/triblock blends as the elastomeric component in hot-melt pressure-sensitive adhesives. It is further known that adhesive properties and viscosity can be controlled by varying the diblock-to-triblock ratio, varying the styrene content, varying the polymer molecular weight, and varying the block molecular weights within the polymers. Examples of materials that have been used are KRATON™ D 1113, containing 16% styrene and 56% diblock; QUINTAC™ 3433, marketed by Nippon Zeon, containing 55% diblock and 17% styrene; VECTOR™ 4114, containing 42% diblock and 17% styrene; and VECTOR™ 4113 containing 20% diblock and 17% styrene. VECTOR™ 4114 and VECTOR™ 4113 are Dexco products. While these materials have good adhesive properties when tackified and can be used in label-production hot melts, they lack optimum die-cutting properties. Furthermore, their low temperature adhesive properties are not optimum.
U.S. Pat. No. 5,663,228 concerns improving label adhesive die-cutability. But the offered solution is complicated, requires two particular block copolymer resins having certain glass-transition temperatures, and requires the choice of a tackifying resin that, when mixed with the two block copolymers, increases the difference between the two block copolymers' glass transition temperatures. Examples of styrenic copolymers that are used in the adhesive mixtures of U.S. Pat. No 5,663,228 are FINAPRENE™ 1205 available from Fina and KRATON™ 1107 available from Kraton Polymers.
U.S. Pat. No. 5,412,032 concerns linear SIS triblock/diblock copolymers that can improve label die-cutting. This is accomplished using block copolymers with a styrene content from 18-24 wt %, a polystyrene block molecular weight from 25,000-35,000, an overall molecular weight from 280,000 up to 520,000, and a coupling efficiency of 20%-40%. The coupling efficiency corresponds to the overall copolymer's triblock content.
U.S. Ser. No. 60/214,308, describes adhesive systems with improved die-cutting performance obtained by optimizing a diblock/triblock blend. We have now found that these improved properties may be obtained with a tetrablock and/or pentablock polymer, thus enabling a single polymerization reaction.
Recognizing that hot-melt, die-cutting performance has to be improved, we analyzed the mechanical and physical aspects of the die-cutting process.
Surprisingly, die-cutting involves relatively low deformation rates and involves pushing the adhesive to the side of the cut line rather than involving a sharp cutting action. In successful die-cutting, the adhesive must creep when subjected to knife action, flow away from the cut point, and not reform over the cut line.
In light of the above, we have found that, for good die-cutting, a styrenic, tetra or pentablock, copolymer adhesives should fulfill the following criteria:                G′ at room temperature monotonically decreasing with frequency at frequencies below the glass transition region (typically <10 rad/s), down to a constant storage modulus plateau at the lowest frequencies. The storage modulus plateau is preferably lower than 10000 Pa, more preferably lower than 7000 Pa and most preferably lower than 4000 Pa.        G′ should intersect a value of 10000 Pa at a frequency preferably higher 20 than 0.001 rad/s; more preferably higher than 0.005 rad/s; and most preferably higher than 0.01 rad/s.        The loss factor Tan δ (defined as the ratio G″/G′) is preferably between 0.2 and 1; more preferably between 0.4 and 1; and most preferably between 0.6 and 1, at the frequency at which the storage modulus intersects a value of 10,000 Pa when measured at 20° C.        
Altogether, both the surprisingly low deformations rates involved in the die-cutting process, as well as the required adhesive flow during die-cutting, explains why water-based acrylic adhesives behave better than their triblock (e.g., SBS or SIS) counterparts. These two systems provide good examples of good and bad die-cutting behavior respectively.
Viscoelastic behavior of hot-melt adhesives at a given temperature is conveniently captured by the two dynamic moduli known as G′ and G″: the loss modulus G″ indicating the viscous behavior, and the storage modulus G′ indicating elastic behavior. The ratio of G″ and G′ is known as the loss factor Tangent delta (Tan δ).
The finding that the cutting mechanism pushes the adhesive away from the cut line rather than sharply cutting it, calls for a less elastic adhesive so that it permanently flows away from the cut line. Emphasis should be put on the low frequency behavior because of the knife's surprisingly small vertical velocity during die-cutting.
Dynamic mechanical analysis of acrylic systems shows indeed that the storage modulus G′ continuously decreases with frequency, indicating no constant plateau at low frequencies. At the same time, there is a relatively high loss modulus G″ at low frequency, essentially overlaying G′. This amplifies the adhesive's tendency to permanently deform and flow under stress, as shown in FIG. 3. On the other hand, similar analysis of pure triblock adhesives shows a constant and relatively high plateau modulus G′ (>10000 Pa) in the low frequency region, much higher than the loss modulus G″. This reflects the adhesive's undesirable tendency to recover from deformation during die-cutting.